1. Field of the Invention
The invention relates to an apparatus for retaining a winding on the stator of an electrical machine and in particular a reluctance machine.
2. Description of Related Art
Switched reluctance machines are well known. Such machines comprise a generally cylindrical stator and, rotatably mounted therein, a rotor. The stator includes a plurality of inwardly directed poles extending the axial length of the stator. Typically, each pole has an energizing coil, the coils being connected in some predetermined pattern to form the stator winding. Adjacent poles define an axially extending slot or channel in which the coil side is retained. The rotor comprises a plurality of radially outwardly projecting poles. When used as a machine, the stator is excited by passing a current through the winding in sequence as the rotor rotates relative to the stator.
A problem that has arisen with known arrangements is that of holding the winding securely in place. If the winding is loose, then it may foul the rotor and cause damage. If the winding is able to vibrate, then it may, over a period of time, abrade the insulation surrounding it and give rise to an insulation failure. The larger the machine, the more difficult it is to secure the windings, since they are heavier and the electromagnetic forces acting on them are greater. If the coils are not firmly held in place against the side of the pole and against the base of the slots formed between the poles, the thermal transfer between the coil and the stator is poor and the ability of the winding to dissipate heat is impaired. This can restrict the output from the machine. In the larger sizes of machines, the copper conductors are of substantial cross section and are not easily forced into place. Any small variations in the coils can make it difficult to insert them fully into the slots and ensure that they are firmly held in place.
In some forms of electrical machine, the slots between the poles of the stator are open, making access straightforward. However, the fact that the slots are open also means that the coils have to be secured in place. It is known to insert slot covers across the mouth of the slot but, unless notches are provided in the sides of the adjacent poles, it is difficult to prevent the slot covers from slipping out into the bore of the stator and fouling the rotor. While notches are sometimes acceptable in small machines, their presence in larger machines causes a disturbance in the flux path which can lead to excessive losses. In addition, because the notches are necessarily small, the dimensions of the slot cover must be carefully controlled to ensure the covers fit correctly.
A further known approach to securing the winding in place is to impregnate the winding with suitable electrically insulating and thermally conducting thermosetting or otherwise curable material, and to locate accurately the winding while the material sets. The preferred method is vacuum pressure impregnation (VPI). This approach is complex and time-consuming and is not suitable for mass production.
A further known approach has been to provide a floating ring of insulating material such as TUFNOL brand synthetic resin-bonded fabric around the exterior of the stator and to hold the coils in place using loops extending between the floating ring and portions of each coil extending beyond each axial end of the stator. Once again this approach is very laborious and time-consuming, and difficult to mechanize.
A further problem is encountered when initially inserting the coils into the slots in the stator. One method of inserting the coils is to rotate the stator until a pole to be wound is at the bottom and then to mount a pre-wound coil over the pole, tapping it into place. Once one coil is mounted, the stator is rotated until the next pole is at the bottom. Tapping of a subsequently fitted coil onto the next pole, however, tends to loosen or even dislodge other coils already in place. Further problems arise when pressure is applied to one axial end of the coil at a time, in which case, as a result of the natural resilience of the coil, the other axial end of the coil will have a tendency to spring out of position.